In recent years, along with the spread of mobile electronic devices such as mobile phones and notebook-sized personal computers, development of smaller and lighter nonaqueous electrolyte secondary batteries having a high energy density has been strongly demanded.
Additionally, development of high power secondary batteries as batteries for electric automobiles typified by hybrid automobiles has been strongly demanded.
The secondary batteries that meet such demands are exemplified by lithium ion secondary batteries. Lithium ion secondary batteries include a negative electrode, a positive electrode, an electrolytic solution and the like, in which a material into and from which lithium can be inserted and desorbed has been used as an active material for the negative and positive electrodes.
Research and development of the lithium ion secondary batteries have been extensively carried out at present, and in particular, lithium ion secondary batteries in which a layer or spinel type lithium metal composite oxide is used as a positive electrode material can achieve a voltage as high as 4V; therefore, practical applications thereof as batteries having a high energy density have been accelerated.
As cathode active materials for use in such lithium ion secondary batteries, proposed are lithium composite oxides such as a lithium nickel composite oxide (LiNiO2), a lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), and a lithium manganese composite oxide (LiMn2O4). Lithium cobalt composite oxides (LiCoO2) are currently in vogue because they can be comparatively easily synthesized.
On the other hand, lithium nickel composite oxides have been receiving attention recently because they have a larger capacity compared with the lithium cobalt composite oxides, and moreover, batteries having a high energy density can be produced at a low price therewith.
Unfortunately, such advantageous lithium nickel composite oxides are inferior in point of thermal stability in a state of charge to lithium cobalt composite oxides. That is, pure lithium nickel dioxides cannot have been used for practical batteries because problems lie in safety of thermal stability or the like, charge-discharge cycle characteristics, and the like. This is because stability of a crystal structure thereof in a state of charge is lower than that of the lithium cobalt composite oxide.
In order to solve the problems of the lithium nickel composite oxides, in general, transition metal elements such as cobalt, manganese, iron or different types of elements such as aluminum, vanadium, tin are substituted for a part of nickel to stabilize the crystal structure in the state of charge with lithium desorbed, thereby obtaining lithium nickel composite oxides with excellent safety and charge-discharge cycle characteristics as a cathode active material (For example, see Non-Patent Literature 1 and Patent Literature 1).
Additionally, Patent Literature 2 discloses a cathode active material represented by LiNi1.x.yCoxTiyO2 wherein 0<x≤0.20, 0<y≤0.07 and including a hexagonal lithium-containing composite oxide with a layer structure. In the cathode active material, a site occupancy of metal ions except lithium is 5% or less in a 3a site if each site of 3a, 3b and 6c in the lithium-containing composite oxide is represented by [Li]3a[Ni1.x.yCoxTiy]3b[O2]6c.
It is also described that the cathode active material has excellent cycle characteristics and can improve thermal stability of batteries without loss of an initial capacity thereof.
Further, a technique to carry out a washing process after firing has been developed for improving thermal stability and capacity of the lithium nickel composite oxides.
Patent Literature 3 discloses a technique to wash with water fired powder represented by a compositional formula: LiNi1-aMaO2 wherein 0.01≤a≤0.5 and M represents at least one element selected from a transition metal element except Ni, a group 2 element and a group 13 element.
It is also described that the washing process can sufficiently remove impurities and by-products adhered to a surface of the fired powder to improve thermal stability and capacity.
Recently, lithium ion secondary batteries have been used for applications required for an instant high current such as batteries for hybrid electric vehicles, and therefore, a high power thereof has been required.
In Patent Literature 2, consideration with respect to increasing an output is not described when the cathode active material is used for a positive electrode of a secondary battery. Moreover, a performance of the cathode active material described in Patent Literature 2 is not sufficient when it is used as secondary batteries for an application required for an instant high current.
The technique described in Patent Literature 3 is regarded to improve a property of the lithium nickel composite oxide by the washing process, however, the washing process unfortunately causes damage on a surface of the lithium nickel composite oxide, resulting in deterioration of output characteristics.
As described above, cathode active materials have not been developed for the present which are capable of a high power suitable for an application required for an instant high current such as batteries for hybrid electric vehicles. Therefore, the development of such cathode active materials is required.